UNITED NATIONS (Reuters) - Twenty human rights groups urged United Nations chief Ban Ki-moon on Wednesday to put a Saudi Arabia-led military coalition back on a U.N. blacklist for killing and maiming children in Yemen because the evidence against it was “overwhelming.”

A soldier from the Saudi-led coalition walks on a street pavement in Yemen's southern port city of Aden September 27, 2015. REUTERS/Faisal Al Nasser

The United Nations removed the coalition from its annual blacklist on Monday pending a joint review by the pair. Riyadh, a key U.N. donor, had threatened to cut off funding, diplomatic sources said on Tuesday. Saudi Arabia denied using threats.

The letter to Ban on Wednesday, signed by groups including Human Rights Watch, Amnesty International and Oxfam, criticized Ban, who steps down at the end of the year, saying he capitulated to Saudi Arabia and tainted his legacy.

“If the Saudi-led Coalition wants to be removed from the list, it should stop killing and maiming children and bombing schools and hospitals in Yemen — the violations for which it was listed,” the groups wrote.

The U.N. report, released last Thursday, said the coalition was responsible for 60 percent of child deaths and injuries in Yemen last year, killing 510 and wounding 667, and half the attacks on schools and hospitals.

“The responsibility of the Saudi-led coalition for grave violations against children in many of these attacks is not in doubt,” the rights groups wrote. “The evidence of grave violations against children in Yemen by the Saudi-led Coalition is overwhelming.”

Saudi U.N. Ambassador Abdallah Al-Mouallimi has described the U.N. figures as “wildly exaggerated.”

The Saudi-led coalition began a military campaign in Yemen in March last year with the aim of preventing Iran-allied Houthi rebels and forces loyal to Yemen’s ex-President Ali Abdullah Saleh from taking power.

Some 6,000 people, about half of them civilians, have been killed in Yemen since last March, according to the U.N.

The rights groups said Ban’s decision to remove the coalition from the U.N. list undermined “an invaluable tool in efforts to curb violations against children in armed conflict.”

“The list creates pressure on parties to armed conflict to comply with international law. Over 20 governments and armed groups have signed U.N. action plans and taken steps to end violations against children in order to be considered for ‘de-listing’,” they wrote.

Last year, the United Nations left Israel and the Palestinian militant group Hamas off the blacklist, after they had been included in an earlier draft, but criticized Israel over its 2014 military operations.